Thomas T. Veblen

Widespread increase of tree mortality rates in the western United States.

Persistent changes in tree mortality rates can alter forest structure, composition, and ecosystem services such as carbon sequestration. Our analyses of longitudinal data from unmanaged old forests in the western United States showed that background (noncatastrophic) mortality rates have increased rapidly in recent decades, with doubling periods ranging from 17 to 29 years among regions. Increases were also pervasive across elevations, tree sizes, dominant genera, and past fire histories. Forest density and basal area declined slightly, which suggests that increasing mortality was not caused by endogenous increases in competition. Because mortality increased in small trees, the overall increase in mortality rates cannot be attributed solely to aging of large trees. Regional warming and consequent increases in water deficits are likely contributors to the increases in tree mortality rates.

The Interaction of Fire, Fuels, and Climate across Rocky Mountain Forests

Abstract Understanding the relative influence of fuels and climate on wildfires across the Rocky Mountains is necessary to predict how fires may respond to a changing climate and to define effective fuel management approaches to controlling wildfire in this increasingly populated region. The idea that decades of fire suppression have promoted unnatural fuel accumulation and subsequent unprecedentedly large, severe wildfires across western forests has been developed primarily from studies of dry ponderosa pine forests. However, this model is being applied uncritically across Rocky Mountain forests (e.g., in the Healthy Forests Restoration Act). We synthesize current research and summarize lessons learned from recent large wildfires (the Yellowstone, Rodeo-Chediski, and Hayman fires), which represent case studies of the potential effectiveness of fuel reduction across a range of major forest types. A “one size fits all” approach to reducing wildfire hazards in the Rocky Mountain region is unlikely to be effective and may produce collateral damage in some places.

CLIMATIC AND HUMAN INFLUENCES ON FIRE REGIMES IN PONDEROSA PINE FORESTS IN THE COLORADO FRONT RANGE

In the northern Colorado Front Range, fire suppression during the 20th cen- tury is believed to have created a high hazard of catastrophic fire in ponderosa.pine (Pinus ponderosa) forests. Since the early 1990s, resource managers have increased the use of prescribed fires to re-create fire regimes and forest structures similar to those of the pre- Euro-American settlement period in order both to reduce fire hazard and to improve forest health. To improve understanding of historical fire regimes, we conducted a study of fire history along an elevational gradient from -1830 to 2800 m in ponderosa pine forests in the northern Front Range. Fire-scar dates were determined from 525 partial cross sections from living and dead trees at 41 sample sites. Fire frequencies and fire intervals were analyzed in relation to changes in human activities and interannual climatic variability as recorded in instrumental climatic records and tree-ring proxy records. Prior to modern fire suppression, the low elevation, open ponderosa pine forests of the northern Front Range were characterized by frequent surface fires, similar in frequency to many other ponderosa pine ecosystems in the West. In contrast, in higher elevation forests (above -2400 m) where ponderosa pine is mixed with Douglas-fir (Pseudotsuga menziesii) and lodgepole pine (Pinus contorta), the fire regime was characterized by a much lower fire frequency and included extensive stand-replacing fires as well as surface fires. In the mid-1800s there was a marked increase in fire occurrence that can be related both to Euro- American settlement and increased climatic variability. This episode of increased fire left a legacy of dense, even-aged stands in higher elevation ponderosa pine forests, whereas increased stand densities in low elevation forests are attributed mainly to fire exclusion during the 20th century. Warmer and drier spring-summers, indicated in instrumental climatic records (1873- 1995) and in tree-ring proxy records of climate (1600-1983), are strongly associated with years of widespread fire. Years of widespread fire also tend to be preceded two to four years by wetter than average springs that increase the production of fine fuels. Alternation of wet and dry periods over time periods of 2-5 years is conducive to fire spread and is strongly linked to El Niiio-Southern Oscillation (ENSO) events. The warm (El Niiio) phase of ENSO is associated with greater moisture availability during spring that results in a peak of fire occurrence several years following El Ninlo events. Conversely, dry springs associated with La Nifia events were followed by more widespread fire during the same year. The 1600-1920 fire-scar record indicates that individual years during which high per- centages of the 41 sample sites synchronously recorded fire have occurred at least several times per century. The association of these years of widespread fire with very strong ENSO events demonstrates the importance of ENSO-related climatic variabililty in creating ex- treme fire hazard at a landscape scale.

Plant succession : theory and prediction

Prologue. The nature of vegetation dynamics D.C. Glenn-Lewin, E. van der Maarel. Establishment, colonization and persistence A.G. van der Valk. Community structure and ecosystem properties R.K. Peet. Regeneration dynamics T.T. Veblen. From population dynamics to community dynamics: modeling succession as a species replacement process R. van Hulst. Statistical models of succession M.B. Usher. Individual-based models of forest succession D.L. Urban, H.H. Shugart. Climate change and long-term vegetation dynamics I.C. Prentice. Epilogue.

Disturbance regime and disturbance interactions in a Rocky Mountain subalpine forest

The spatial and temporal patterns of fire, snow avalanches and spruce beetle outbreaks were investigated in Marvine Lakes Valley in the Colorado Rocky Mountains in forests of Picea engelmannii, Abies lasiocarpa, Pseudotsuga menziesii and Populus tremuloides. Dates and locations of disturbances were determined by dendrochronological techniques. A geographic information system (GIS) was used to calculate areas affected by the different disturbance agents and to examine the spatial relationships of the different disturbances. In the Marvine Lakes Valley, major disturbance was caused by fire in the 1470s, the 1630s and the 1870s and by spruce beetle outbreak in c. 1716, 1827 and 1949 (...)

Contingent Pacific–Atlantic Ocean influence on multicentury wildfire synchrony over western North America

Widespread synchronous wildfires driven by climatic variation, such as those that swept western North America during 1996, 2000, and 2002, can result in major environmental and societal impacts. Understanding relationships between continental-scale patterns of drought and modes of sea surface temperatures (SSTs) such as El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) may explain how interannual to multidecadal variability in SSTs drives fire at continental scales. We used local wildfire chronologies reconstructed from fire scars on tree rings across western North America and independent reconstructions of SST developed from tree-ring widths at other sites to examine the relationships of multicentury patterns of climate and fire synchrony. From 33,039 annually resolved fire-scar dates at 238 sites (the largest paleofire record yet assembled), we examined forest fires at regional and subcontinental scales. Since 1550 CE, drought and forest fires covaried across the West, but in a manner contingent on SST modes. During certain phases of ENSO and PDO, fire was synchronous within broad subregions and sometimes asynchronous among those regions. In contrast, fires were most commonly synchronous across the West during warm phases of the AMO. ENSO and PDO were the main drivers of high-frequency variation in fire (interannual to decadal), whereas the AMO conditionally changed the strength and spatial influence of ENSO and PDO on wildfire occurrence at multidecadal scales. A current warming trend in AMO suggests that we may expect an increase in widespread, synchronous fires across the western U.S. in coming decades.

FIRE HISTORY IN NORTHERN PATAGONIA: THE ROLES OF HUMANS AND CLIMATIC VARIATION

The effects of humans and climatic variation on fire history in northern Patagonia, Argentina, were examined by dating fire scars on 458 trees at 21 sites in rain forests of Fitzroya cupressoides and xeric woodlands of Austrocedrus chilensis from 39° to 43° S latitude. Climatic variation associated with fires was analyzed on the basis of 20th-century observational records and tree ring proxy records of climatic variation since approximately AD 1500. In the Austrocedrus woodlands, fire frequency increases after about 1850, coincident with greater use of the area by Native American hunters. Increased burning, particularly in the zone of more mesic forests, is also strongly associated with forest clearing by European settlers from about 1880 to the early 1900s. The marked decline in fire frequency during the 20th century coincides with both the demise of Native American hunters in the 1890s and increasingly effective fire exclusion. Strong synchroneity in the years of widespread fire at sample sites disperse...

The response of subalpine forests to spruce beetle outbreak in Colorado

Spruce beetle (Dendroctonus rufipennis Kirby) outbreaks are important dis- turbances affecting subalpine forests of Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa), and lodgepole pine (Pinus contorta) in the southern Rocky Mountains. However, little is known about the influences of these outbreaks on overall forest dynamics. We used age-structure analyses and dendrochronological techniques to investigate the effects of a major spruce beetle outbreak on stand composition, dominance, tree age and size structures, radial growth, and succession in subalpine forests in Colorado. This out- break, which occurred in the 1940s, caused a shift in dominance from spruce to fir and a reduction in average and maximum tree diameters, heights, and ages. The outbreak did not favor new seedling establishment of the seral lodgepole pine. Thus, in seral stands spruce beetle outbreak accelerates succession towards the shade-tolerant tree species. The predominant response to the outbreak was the release of previously suppressed small-diameter spruce (not attacked by the beetle) and subalpine fir (a non-host species). Following the 1940s outbreak, growth rates of released trees remained high for >40 yr. The relative increases in growth rates were similar for both species. Both spruce and fir will continue to codominate the affected stands. The predominance of accelerated growth following a spruce beetle outbreak, instead of new seedling establishment, is a major contrast to the pattern of stand development following fire. In some Colorado subalpine forests the effects of disturbance by spruce beetle outbreaks appear to be as great as those due to fire.

EFFECTS OF CLIMATIC VARIABILITY ON FACILITATION OF TREE ESTABLISHMENT IN NORTHERN PATAGONIA

Facilitation of tree establishment by nurse shrubs, which ameliorate otherwise unfavorable microenvironmental conditions, is a widely studied phenomenon. However, relatively little is known about how facilitative influences change in relation to interannual climatic variability. In northern Patagonia, Argentina, we examined influences of potential nurse shrubs on the establishment of the conifer Austrocedrus chilensis and assessed the significance of those influences to establishment during years of contrasting climate. We also experimentally investigated the effects of nurse shrubs and different water availability on tree seedling emergence and survival. A strong spatial association of Austrocedrus juveniles with shrubs, both beneath shrub canopies and near shrub canopies, indicates that shrubs favorably influence tree regeneration and that in some habitats and time periods nurse plants appear to be required for successful tree seedling establishment. Protection from direct sunlight was the main factor c...

Fire and climatic change in temperate ecosystems of the Western Americas

Preface.- Section 1. Methods and Models: Fire-History Reconstructions Based on Sediment Records from Lakes and Wetlands.- Simulation of Landscape Fire, Climate and Ecosystem Dynamics.- Simulation of Effects of Climatic Change on Fire Regimes.- Section 2. North America: Fire Regimes and Climatic Change in Canadian Forests.- Fires and Climate in Forested Landscapes of the US Rocky Mountains.- Tree-Ring Reconstructions of Fire and Climate History in the Sierra Nevada of California and Southwestern United States.- The Influence of Climate and Land Use on Historical Surface Fires in Pine-Oak Forests, Sierra Madre Occidental, Mexico.- Impact of Past, Present and Future Fire Regimes on North American Mediterranean Shrublands.- Section 3. South America: Fire History and Vegetation Change in Northern Patagonia, Argentina.- Influences of Climate on Fire in Northern Patagonia, Argentina.- Fire Regimes and Forest Dynamics in the Lake District of south central Chile.- Fire History in Central Chile: Tree-Ring Evidence and Modern Records.- Holocene Fire Frequency and Climate Change at Rio Rubens Bog, Southern Patagonia.- Regeneration Potential of Chilean Matorral after Fire: An Updated View.- Section 4. Practical Implications: Management Implications of Fire and Climate Changes in the Western Americas.